NUMERICAL STUDY OF SEVERE CONVECTION ON APRIL 19, 1996


Brian F. Jewett, Bruce D. Lee(*) and Robert B. Wilhelmson
Department of Atmospheric Sciences and National Center for Supercomputing Applications (NCSA)
University of Illinois at Urbana-Champaign
112 ATMOS SCI Building, 105 S. Gregory St., Urbana, IL 61801
Phone: (217) 333-3957 FAX: (217) 244-4393
email: bjewett@ncsa.uiuc.edu , bw@ncsa.uiuc.edu
* Dept. of Earth Sciences, University of Northern Colorado
email: bdlee@bentley.unco.edu

On April 19, 1996 an outbreak of tornadic storms occurred in central and northern Illinois and parts of Iowa, Indiana and Wisconsin. The single-day total of 30 tornadoes was the largest ever reported in Illinois. Although most of the tornadoes were short-lived, some reached F2-F3 intensity, with one fatality in the state. The storms were associated with an upper-level jet streak and a deepening surface cyclone moving from Iowa into western Illinois. The intensifying cyclone was accompanied by strong southeast surface winds and strong vertical shear and thus an environment conducive to severe thunderstorms.

The severe thunderstorms occurred along both the surface warm front, which extended from Iowa through northern and eastern Illinois, and a north-south surface dryline which moved into Illinois from Missouri. Preliminary analysis of the WSR-88D data indicates that the storms along the dryline underwent a pattern of cell splitting and merging prior to intensifying into a broken line of tornadic storms that moved across central Illinois.

We are modeling the larger-scale cyclone as well as the severe squall line with the NCAR/Penn. State MM5 mesoscale model. Our research goals in this case include understanding the thunderstorm cell-cell interaction which occurred prior to formation of tornadoes, and the extent to which this evolution was necessary for the development of sustained severe thunderstorms. We will also use the MM5 model solution to generate vertical soundings along the mesoscale boundaries (warm front and dryline). These soundings will be introduced into the COMMAS model, where isolated storm simulations would be performed. The differences between the MM5 and COMMAS simulations should reveal the role of the surface boundaries in the actual thunderstorm evolution.